Thank you all for your concern and suggestions.
Yes, it has an adjustable one. I tightened it as much as possible when I started running on wood. The remaining small gap is most likely plugged with soot by now.
Ha, seems like a combination + a few more reasons. One of them being a flimsy dash board for my idle string nob
Anyway, I did some additional cleaning and exercised the butterflies a bit more. Also I mounted a sturdier return spring.
I can keep the woodgas idle down around 800 rpm again. On gasoline I have to turn up the idle higher not to stall. I’ve always had ruff gasoline idle.
However the smallest brass butterfly does seem to be affected by corrosion on the leading edge.